A judge will rule this month on whether the Government’s decision in January 2009 to add a third runway at Heathrow was lawful.

In one of a series of grounds put forward, lawyers representing a coalition of local councils and green groups told the High Court this week that the Government should have re-consulted on its decision in the light of the Climate Change Act.

Lord Justice Carnwath asked the Government’s lawyer why the decision was announced before the Climate Change Committee’s report on how aviation could meet its target of reducing emissions to 2005 levels in 2050 had been received.

The CCC’s report, which was presented in December 2009, used an average carbon cost of £200 per tonne compared to the Government’s figure of £63. The claimants’ lawyer Nigel Pleming QC argued that this change alone would have brought the economic benefits from expansion almost back to zero.

The Court also heard that the Government had failed to include the cost of added congestion on the local road and rail network. Transport for London said the calculations showed that while there might be enough capacity on the Piccadilly Line for people to board at an airport terminus, these had not considered whether there would be room on the train at stops like Hammersmith and Earls Court.

The Court was told that that figures showed the Government had estimated carriages accommodating four people to every square metre – with no allowance for their luggage. They had also used an out of date population forecast for the capital. The underestimate was equivalent to 670,000 people – a city the size of Glasgow.

The Court also heard that the Government had based its conclusion that public transport would be sufficient on improvements such as Crossrail and the Piccadilly Line upgrade which were already being planned for a two runway airport. Lord Justice Carnwath asked the Government’s lawyer if this was not ‘double counting’

The Government said during the hearing that the January 2009 decision was not a ‘policy decision’ but more ‘a step along the way.’ It said it intended to have further consultation on a national policy statement taking into account the climate change targets and reopening the economic case for the additional runway.

Hounslow leader Peter Thompson speaking on behalf of the councils said, “This concession is significant. It means the runway is not a done deal and shows why it was so important we went to court to protect our residents’ interests.”

The judgment is expected to clarify how this fresh consultation would be carried out.

The challenge was brought by Hammersmith and Fulham, Hounslow, Hillingdon, Richmond upon Thames, Wandsworth and Windsor & Maidenhead councils with support from Kensington and Chelsea, Transport for London and the Mayor of London.

The councils were joined by the local residents group (Notrag), aircraft noise campaigners HACAN, World Wildlife Fund UK, Campaign to Protect Rural England and Greenpeace. The Royal Society for the Protection of Birds was an expert witness.

The local authorities are all members of the 2M Group which comprises 24 local councils opposed to Heathrow expansion with a combined population of 5 million